Yes, it's easier to plug bits and pieces of Amiga OS 3.9 (+BB2/3/4) ontop of 3.1.4 than vice versa. As bonus, you then also avoid the mostly useless diskfilling bloat and "bling" of 3.9.
Sadly they conflict here and there though, such as IPrefs and locale string IDs, at least. My work-around has been to make locale:catalogs/mylang/3.9, fill them with catalog files from catalogs/mylang/sys of 3.9, and then binary edit the 3.9 binaries to look there. Or vice versa.